package craky.security;

import java.util.zip.Checksum;

public class CRC16_CCITT
  implements Checksum
{
  private static final int[] CRC_TABLE = { 0, 4129, 8258, 12387, 16516, 20645, 24774, 28903, 33032, 
    37161, 41290, 45419, 49548, 53677, 57806, 61935, 4657, 528, 12915, 8786, 21173, 
    17044, 29431, 25302, 37689, 33560, 45947, 41818, 54205, 50076, 62463, 58334, 9314, 
    13379, 1056, 5121, 25830, 29895, 17572, 21637, 42346, 46411, 34088, 38153, 58862, 
    62927, 50604, 54669, 13907, 9842, 5649, 1584, 30423, 26358, 22165, 18100, 46939, 
    42874, 38681, 34616, 63455, 59390, 55197, 51132, 18628, 22757, 26758, 30887, 2112, 
    6241, 10242, 14371, 51660, 55789, 59790, 63919, 35144, 39273, 43274, 47403, 23285, 
    19156, 31415, 27286, 6769, 2640, 14899, 10770, 56317, 52188, 64447, 60318, 39801, 
    35672, 47931, 43802, 27814, 31879, 19684, 23749, 11298, 15363, 3168, 7233, 60846, 
    64911, 52716, 56781, 44330, 48395, 36200, 40265, 32407, 28342, 24277, 20212, 15891, 
    11826, 7761, 3696, 65439, 61374, 57309, 53244, 48923, 44858, 40793, 36728, 37256, 
    33193, 45514, 41451, 53516, 49453, 61774, 57711, 4224, 161, 12482, 8419, 20484, 
    16421, 28742, 24679, 33721, 37784, 41979, 46042, 49981, 54044, 58239, 62302, 689, 
    4752, 8947, 13010, 16949, 21012, 25207, 29270, 46570, 42443, 38312, 34185, 62830, 
    58703, 54572, 50445, 13538, 9411, 5280, 1153, 29798, 25671, 21540, 17413, 42971, 
    47098, 34713, 38840, 59231, 63358, 50973, 55100, 9939, 14066, 1681, 5808, 26199, 
    30326, 17941, 22068, 55628, 51565, 63758, 59695, 39368, 35305, 47498, 43435, 22596, 
    18533, 30726, 26663, 6336, 2273, 14466, 10403, 52093, 56156, 60223, 64286, 35833, 
    39896, 43963, 48026, 19061, 23124, 27191, 31254, 2801, 6864, 10931, 14994, 64814, 
    60687, 56684, 52557, 48554, 44427, 40424, 36297, 31782, 27655, 23652, 19525, 15522, 
    11395, 7392, 3265, 61215, 65342, 53085, 57212, 44955, 49082, 36825, 40952, 28183, 
    32310, 20053, 24180, 11923, 16050, 3793, 7920 };

  private int crc = 65535;

  public long getValue()
  {
    return this.crc & 0xFFFFFFFF;
  }

  public void reset()
  {
    this.crc = 0;
  }

  public void update(int b)
  {
    this.crc = update(this.crc, b);
  }

  public void update(byte[] b, int off, int len)
  {
    if (b == null)
    {
      throw new NullPointerException();
    }
    if ((off < 0) || (len < 0) || (off > b.length - len))
    {
      throw new ArrayIndexOutOfBoundsException();
    }

    this.crc = updateBytes(this.crc, b, off, len);
  }

  public void update(byte[] b)
  {
    this.crc = updateBytes(this.crc, b, 0, b.length);
  }

  private int update(int crc, int b)
  {
    crc = (crc & 0xFF) << 8 ^ CRC_TABLE[(((crc & 0xFF00) >>> 8 ^ b) & 0xFF)];
    return crc;
  }

  private int updateBytes(int crc, byte[] b, int off, int len)
  {
    for (int i = off; i < off + len; i++)
    {
      crc = update(crc, b[i]);
    }

    return crc;
  }
}